The molecular and genetic basis of cancer will be studied using avian leukosis viruses and chickens as a model system. In vivo oncogenicity tests will be used to define viral genes that encode the induction of a high incidence of lymphoma or the induction of a high incidence of osteopetrosis and to define host genes that determine susceptibility to the induction of lymphomas and erythroblastosis. Tumor material acquired from the oncogenicity tests will be used to indentify host genes that are targets for cancer induction by insertional mutagenesis. The organization and expression of these genes will be examined in normal and diseased tissue. A novel model for the life cycle of ALVs that induce a high incidence of osteopetrosis will be tested. DNAs from selected tumors will be analyzed for their ability to transform NIH/3T3 cells.